This application claims the benefit of U.S. Provisional Application Ser. No. 60/299,086 filed Jun. 18, 2001.
The present invention relates to oriented nylon casings, particularly food casings suitable for making sausage, ham in mold products, and D-shaped products.
Tubular films are used as sausage casings for processing and packaging cooked foods including water or steam cooked sausages, such as liver sausage and fleischwurst, hams processed in molds, and “D” shaped foods such as turkey breasts with one flat side.
The selection of films for packaging food products includes consideration of a number of criteria such as cost, abrasion resistance, wrinkle resistance, meat adhesion, dimensional uniformity and stability, stiffness, strength, printability, durability, oxygen and water barrier properties, stretchability, machinability, optical properties such as haze, gloss, and freedom from streaks and gels, and safety for contact with food.
In general, commercial sausage making operations for making water cooked or steam cooked sausages require casings made from materials able to perform well in the following typical process steps:                1. Stuffing with meat emulsion to a uniform diameter;        2. Clipping or otherwise sealing the casing about its circumference to form discrete logs;        3. Cooking the encased sausage to temperatures of at least 65–100° C.;        4. Chilling the cooked encased sausages to 4° C. or less;        5. Optionally cutting the logs into discrete lengths or slices; and        6. Repackaging cut logs or slices by vacuum packaging.        
Sausages so made must be uniform in size and must be wrinkle free.
Other uses for casings include processing hams-in-mold and meat products such as turkey breasts or loaf products. Hams processed in molds are processed by encasing a ham or a portion of one in a casing, cutting the casing to form a loose package around the ham, clipping or sealing the casing end, placing the ham into a mold, squeezing it and closing the lid, cooking the ham in the mold, and releasing it from the mold and ultimately removing the casing. After cooking, ham in mold products must have square corners and must be of certain dimensions to fit into vacuum sealed cans that are sold to the final customer.
“D” shaped foods are similarly processed as in ham in mold, except that once the turkey meat, for example, is encased, it is placed on a tray and then further heat processed. The bottom edge is flattened, giving the processed turkey a “D” shape, thereby imitating the shape of a bone-in turkey breast. A product having a more oval shape, the manufacturer would consider the product unacceptable. A similar situation exists for loaf shaped products, which may later be sliced as deli meats. These products must not become rounded during processing, but rather must retain the desired D or loaf shape.
The various required shapes and wrinkle-free features of the finished products are provided by the use of the correct type of casing.
Various monolayer and multilayer casings have been proposed and used commercially to make processed foods. Moisture barrier properties are important to prevent loss of water during and after cooking. Desirably, these casings will also have low oxygen permeability to avoid discoloration, adverse flavor changes, and oxidation of the food stuff during storage. Liver sausage in particular is easily susceptible to defects when contacted with excessive oxygen, resulting in discoloration,
Furthermore, it is highly desirable to produce an encased cooked food stuff that exhibits a tight fitting casing having few or no wrinkles even after prolonged storage. There should be a minimum of spaces or pockets between the food mass and the inside of the casing since such spaces or pockets promote separation and collection of fats, liquid and gelatinous materials in such spaces which leads to a non-uniform meat or sausage appearance that is unappetizing and undesirable to consumers.
Cellulose casings of, e.g., fiber reinforced regenerated cellulose coated with moisture barrier coatings such as a polyvinylidene chloride copolymer (“PVDC”), e.g., saran, have been commercialized, as have monolayer casings made of PVDC. These casings have excellent oxygen and moisture barrier properties.
Also, commercially available coated cellulosic casings have excellent dimensional uniformity and stability, but are expensive to produce compared to plastic casings. However, use of PVDC has raised environmental concerns due to the difficulties of recycling chlorinated polymers and possible release of chlorinated by-products during incineration. Furthermore, the dimensional stability and uniformity of saran monolayer casings are generally inferior to the cellulosic casings, as saran monolayer casings after cooking and chilling tend to relax, thereby causing a wrinkled appearance.
To ameliorate the aforementioned problems and costs associated with coated cellulosic casings, several polyamide casings have been introduced into the market. Both monolayer and multilayer polyamide casings have been commercialized and both non-shrinkable and shrinkable casings have been made by blown film and oriented film processes.
As discussed in U.S. Pat. No.4,303,711, “single-layer, unstretched plastic casings consisting of higher homopolyamides (polyamide 11 and polyamide 12)” are known as well as plastic casings consisting of such polyamides coextruded in two layers with polyamide 6 as the outer layer. These polyamide casings may be easily made by the blown film technique, but typically suffer from a lack of dimensional stability and uniformity, being deformed upon stuffing so that production of stuffed sausages to a uniform diameter is difficult. The '711 patent further indicates that these unstretched films suffer from an undesirably wrinkled appearance after cooking and chilling.
Various attempts have been made at making stretch oriented polyamide casings. Uniaxially stretched casings which are only stretched in the longitudinal (machine) direction (“MD”) reportedly have the same disadvantages as unstretched casings regarding insufficient dimensional stability, nonuniformity of diameter and excessive wrinkling.
U.S. Pat. No. 4,560,520 (Erk et al.) discloses forming multiaxially stretched, monolayer, polyamide, tubular films, e.g., of nylon 6 or nylon 66 which have elastic properties and which are said to be used for packaging table sausages and boiled sausages. The disclosed films are “fixed thermally” and shrunk after stretching, e.g., by subjecting the tube to controlled shrinkage of at least 15% and at most 40% at temperatures above 90° C. and also subjecting the film to infrared irradiation. This is to produce a nylon casing which does not have shrinkage at temperatures under 90° C. This pre-shrunk casing is used for stuffing with meat emulsion and relies upon its elastic properties to provide wrinkle resistance.
This patent goes on to refer to a “shrinkable multiaxially stretched thermally fixed sausage casing of polyamide”, which remains close fitting but suffers from insufficient resistance to tearing. The assignee of this patent, Naturin-Werk Becker & Company has commercialized several monolayer nylon casings under the trademarks Optan, Betan and Tripan.
Nylon monolayer casings whether made by the blown film process or the stretch oriented film process are disadvantageously sensitive to moisture. As noted above, it is desirable for casings used to package fleischwurst and liver sausage-type products to have low permeability to steam or water. Also, moisture is known to adversely affect the oxygen barrier properties of many nylons causing an undesirable increase in oxygen transmission rates when wet. In view of these disadvantages, attempts have been made to utilize blends of nylon with other materials to enhance properties such as gas and water vapor impermeability. For example, U.S. Pat. No. 4,303,711 discloses a plastic casing made from a mixture of polyamide and ionomer. Also, the company Hoechst AG has commercialized what are believed to be monolayer casings containing blends of polyamide and polyester.
Furthermore, attempts have been made to use nylon in biaxially stretched multilayer casing to overcome these disadvantages. For example, U.S. Pat. No. 4,888,223 discloses 2 to 5 layer heat shrinkable tubular structures all having polyamide in either the outer layer or core layer and having a polyolefin inner layer that is corona treated for meat adhesion.
Also, U.S. Pat. No. 4,855,183 discloses a multilayer tubular shrink film having a polyamide inner layer which is irradiated to promote meat adhesion and which has additional polyolefinic layers which may comprise materials such as EVA, EMA, EEA, LLDPE, VLDPE, LDPE, HDPE or MDPE.
Disadvantageously, the above '223 and '183 patents disclose structures that require a corona treatment or irradiation step to enhance meat adhesion thereby requiring additional equipment, processing time and/or cost.
U.S. Pat. No. 5,185,189, issued on Feb. 9, 1993 to Stenger, discloses a multilayer casing which may consist of a three layer coextruded and biaxially oriented tube which, if desired, may be “thermofixated”. A structure having inner and outer polyamide layers separated by a middle layer of polyolefin, blended with or coated with an adhesion imparting component, is disclosed for use as a sausage casing with low permeability to steam and oxygen. The middle layer is preferably a polyolefin blended with an adhesion imparting component in a portion which in general is 5–50 weight % based on the polymer blend present in the middle layer. According to this document, the portion of the adhesion imparting component is preferably 10 to 35% by weight but “should be kept as low as possible”. This adhesion component is viewed as being required to prevent layer separation during cooking in hot water. Useful adhesion imparting agents are disclosed as including polyolefin resin modified with functional groups such as vinyl acetate, acrylic acid and methacrylic acid, as well as their esters and salts, and furthermore, ethylenically unsaturated carboxylic anhydride groups. These casings are oriented by biaxial stretching. To improve dimensional stability after stretching, the casing is annealed to produce a material having a shrinkage of less than 20%, preferably less than 15% in both vertical and horizontal directions, at temperatures of up to 90° C. Elastic behavior and shrinkage during drying are relied upon to provide a purportedly tight wrinkle free fit.
Unfortunately, orientation of seamless tubes of nylon by biaxial stretching is difficult. Extrusion and orientation of multilayer tubes, especially coextruded tubes, containing mixed layers of polyamides and other materials having different melting points, melt viscosities, and a different affinity for water can be very difficult. For example, U.S. Pat. No. 4,892,765 (Hisazumni et al.) notes that although it is desirable to extrude films for packaging hams and sausages in tubular form, it is difficult to make a stretched tubular polyamide film of uniform thickness. This patent also notes that layer adhesion becomes weak when multilayer, polyamide films are stretched. Hisazumi et al. disclose production of a heat shrinkable multilayer film having a core layer of a PVDC attached to opposing polyamide layers (e.g., of nylon 6/66 copolymer) by opposing adhesive layers. This film is made using an orientation process which utilizes water to soften and plasticize the nylon to a degree sufficient to allow or facilitate orientation. The orientation processes employed for nylon multilayer casing have tended to involve complicated apparatus and processing such as that found in U.S. Pat. No. 4,886,634. Cook-in pouches developed by Viskase Corp. and known as “Cook-Tite” are made of layers arranged as polyolefin/adhesive/ethylene vinyl alcohol/adhesive/polyolefin. Raw meats are placed inside such pouches, sealed and then subjected to cooking.
U.S. Pat. No. 5,698,279, issued on Dec. 16, 1997 to Vicik, describes a multilayer tubular film comprising the structure of inner and outer layers of polyamide, with a core layer of a blend of EVA and another polyolefin such as ethylene methacrylic acid copolymer, providing casing with good oxygen barrier properties.
U.S. Pat. No. 5,747,124, issued on May 5, 1998 to Pophusen, describes a tubular film having at least four layers, where the inner and outer layers are polyamides, and the enclosed two layers are oxygen barrier layer, such as EVOH, and an polyolefin, having a quotient of tear resistance of TD/MD of less than or equal to 0.85.
U.S. Pat. No. 5,985,386, issued on Nov. 16, 1999 to von Widdem claims a multilayered, biaxially oriented tubular sausage casing made of a film of at least four layers where the inner and outer layers are a polyamide or blend of a number of polyamides and where at least either the inner or outer layer polyamide also is blended with another polyamide, copolyamide or polyolefin copolymer; and where there is a center water barrier layer of a variety of polyolefins; and another center layer of ethylene vinyl alcohol. In its examples, '386 also discloses a feature of this film whereby the inner polyamide layer(s) are thicker than the outer layer, which may at least partially account for the barrier features of this film. This provides for a casing that has oxygen and moisture barrier properties suitable for longer storage life for the encased sausages. No teaching is made of the use of this film to produce ham in mold or D shaped products.
U.S. Pat. No. 6,194,040 B1, issued on Feb. 27, 2001 to Delius et al., also describes a casing useful for longer term storage of encased sausage. A tubular, biaxially-oriented casing having four layers, where the outer layer is a mixture of two polyamides, a first intermediate layer is ethylene vinyl alcohol blended with a copolyamide, an olefinic polymer, or an ionomer, a second intermediate layer of an polyolefin, and an inner layer of an aliphatic polyamide.
In summary, although several of the aforementioned plastic casing products have gained varying degrees of commercial acceptance in different market segments, their advantage compared to the traditional cellulosic casing has been chiefly one of cost with the problems of dimensional stability, uniformity of diameter, and wrinkling being persistent concerns.
Prior art fiber reinforced cellulose casings coated with moisture barrier coatings perform well in processing water/steam cooked sausages such as fleischwurst and liver sausage. However, the high cost of manufacture of such casings has led casing manufacturers to search for less expensive alternatives. Thermoplastic films of various compositions have been suggested and some have found varying degrees of success in various segments of the market. Thermoplastic sheet film has been made into a tube by seaming, but this is a difficult process which produces a casing having a seamed area which may undesirably differ in appearance and performance relative to an unseamed casing.
Seamless tubular thermoplastic casings have been made which overcome the objections to seamed casings. Various materials have been employed, but materials containing chlorinated polymers have been objected to for environmental reasons among others. Seamless polyamide casings have been made of blown film, however these casings tend to have poor performance with respect to wrinkling, uniformity of diameter, and dimensional stability. Seamless biaxially oriented multilayer films have also been made, However such films have been difficult to produce, requiring special blend formulations and structures or complicated equipment and procedures.
Therefore, it is an object of the present invention to provide a multilayer, biaxially stretched, heat shrinkable, thermoplastic film useful as a casing for foodstuffs of both regular and irregular shapes, needing moisture and oxygen barriers during processing and afterward, such as sausages or ham in mold products, which includes among its desirable properties one or more, of the following:                a) sufficient flexibility and softness to facilitate shirring and subsequent stuffed log formation or irregular shapes by gathering of the casing and clipping to form the ends;        b) resistance to permanent deformation during stuffing, cooking and chilling, and maintenance of a symmetrical cylindrical shape, if needed, with a minimum of curvature or bulging;        c) an acceptable, to the final customer, level of change of height of the packaging between the preprocessed and the postprocessed products, when the products must have a final D shaped configuration.        d) an ability to adhere to both the regularly or irregularly shaped food during expansion and contraction of the food during cooking and chilling;        e) resistance to bursting or tearing during stuffing, when pressure is applied to force the encased foodstuff into a mold, during cooking at elevated temperatures, and during subsequent handling;        f) resistance to wrinkling during processing and handling;        g) little or no moisture loss during cooking and storage, i.e., high cooking yield;        h) resistance to passage of oxygen in order to prevent spoilage; and        i) an ability to be cut or sliced easily without edge curling or splitting, and to be removed without damaging the surface of the foodstuff        
It is a further object of the present invention to provide a tubular film having a unique combination of shrink, mechanical strength and barrier properties suitable for use as a foodstuff casing, where the foodstuffs include both regularly shaped chubs and irregularly shaped foodstuffs such as meat parts.
It is a further object of the present invention to provide a polyamide sausage casing having a polyamide inner layer which adheres to meat without requiring addition of starch based additives or treatment with electron beam irradiation or corona discharge.
It is a further object of the present invention to provide a multilayer oriented structure having sufficient shrinkage values and shrink force values to provide good conformation of the casing to the filling after cooking, chilling and storage.
It is a further object of the present invention to provide an improved biaxially stretched, heat shrinkable polyamide multilayer casing, that in addition, can accept coloring in a discrete layer.